Prepare the active starter: Prepare 100 grams of active sourdough starter by mixing 20 grams of existing starter, 40 grams of flour, and 40 grams of water. It will be ready to use once it has doubled in size and is bubbly (about 4-6 hours).
Make the dough: In the bowl of a stand mixer, combine the active sourdough starter, milk, melted butter, sugar, garlic salt, and parsley with the paddle attachment. Mix on medium speed as you gradually add the flour one cup at a time until just combined.
Knead the dough: Switch to the dough hook attachment and knead on medium speed for 7-10 minutes.
Make the cheesy garlic bombs: Prepare a cast iron skillet (or other baking dish) with parchment paper. Divide the dough into 10 equal dough balls. To ensure accuracy, measure the ball of dough (about 850 grams) and divide by 10, bringing each dough ball to 85 grams.
Stuff the garlic bombs: Divide the mozzarella cheese into 10 equal pieces. Use a rolling pin or your hands to flatten out each dough ball and add the mozzarella to the middle. Fold the dough into the center over the cheese, then pinch the dough together. Use your hands to roll the dough into balls.
Chill overnight: Place all the dough balls into the prepared dish. Cover with a damp tea towel and place in the fridge overnight (8-12 hours).
Bake the garlic bombs: When you're ready to bake, preheat the oven to 400℉. Remove the garlic bombs from the fridge. Melt the butter and add the garlic salt and dried parsley. Brush the seasoned melted butter over the top of the garlic bombs. Bake for 25 minutes or until they are golden brown on top. Dip in marinara sauce or on their own. Enjoy!